The present invention relates generally to the field of syntheses of methacrylic acid and methacrolein, and more particularly to syntheses of such materials using heteropolyacid catalysts.
Methacrylic acid and methacrolein are chemical species which are fundamental to the plastics industry. Methacrylic species, such as methacrylic esters, are used in huge quantities worldwide for diverse employment in the formulation of structural, coating, aesthetic and other polymerizable resin systems. Accordingly, the efficient synthesis of methacrylic acid and other methacrylic species, such as methacrolein, is greatly desired.
Isobutylene oxide is a readily available material having the requisite carbon atom structure for transformation into methacrylic acid and methacrolein species. Such transformation, however, requires both oxidation and dehydration. Accordingly, catalytic agents which accomplish this transformation in a selective and efficient manner are highly desired.
Heteropolyacids are a recognized class of acids containing large amounts of oxygen and hydrogen, and multiple atoms of one or more elements, such as molybdenum or tungsten, surrounding one or more heteroatoms of another element, such as phosphorus. Polyanions of such acids consist primarily of octahedral MoO.sub.6 or WO.sub.6 groups, so that the conversion of [MoO.sub.4 ].sup.2- or [WO.sub.4 ].sup.2- into polyanions requires an increase in coordination number. Cotton and Wilkinson, "Advanced Inorganic Chemistry", 4th edition, pp. 852-861, Wiley & Sons, N.Y. (1980), disclose that heteropolyanions can be formed either by acidification of solutions containing the requisite simple anions, or by introduction of the hetero element after first acidifying the molybdate or tungstate. As indicated at Table 22-C-2 of Cotton and Wilkinson (pg. 857), various heteropolyanion formula types are known, including the well known Keggin and Dawson structures. Thus, as used herein, the term "heteropolyacids" refers to heteropolyacids and their salts, including heteropolyacids of the Keggin and Dawson structures, as well as organoheteropoly anions and heteropoly blues, as described in the aforementioned Cotton and Wilkinson article at pages 852 through 861, which publication is hereby incorporated by reference as if fully set forth herein.
It has long been known to use various heteropolyacids to catalyze certain organic reactions. For example, in U.S. Pat. No. 4,192,951, discloses vapor phase oxidation procedures utilizing various heteropolyacid catalysts, including heteropolymolybdic catalysts containing vanadium, tungsten, tantalum or niobium act, which catalysts are used in the synthesis of such compounds as maleic acid and acetic acid.
Similarily, in U.S. Pat. No. 4,146,574 entitled "Process For Preparing Heteropoly-Acids" various heteropoly-acid catalysts are disclosed. Such catalysts are described as facilitating the oxidative dehydrogenation of isobutyric acid to methacrylic acid, the oxidative dehydrogenation of methyl isobutyrate to methylmethacrylate and methacrylic acid, the oxidative dehydrogenation of isobutyraldehyde to methacrolein and methacrylic acid, the oxidation of methacrolein to methacrylic acid, and/or the oxidative dehydrogenation of methylisopropyl ketone to methylisopropenyl ketone.
Vapor phase heteropolyacid catalized reactions, such as disclosed in U.S. Pat. No. 4,146,574, are normally conducted using a mixture of gases, such as steam, oxygen, and/or nitrogen which are permitted to contact a catalytic substrate for preselected periods at preselected reaction temperatures.